Surface emitting semiconductor laser

ABSTRACT

A surface emitting semiconductor laser is formed from a substrate having a first mirror formed thereon. The first mirror includes semiconductor layers of a first conductivity type. A second mirror is formed over the substrate and includes semiconductor layers of a second conductivity type. An active region is disposed between the first and second mirrors, with a current confining layer being disposed between the first and second mirrors. A compound semiconductor layer is formed over the second mirror and an electrode is formed on the compound semiconductor layer. A protective film covers the compound semiconductor layer and partially covers the electrode. The electrode is formed by a lift-off process and uses an opening-pattern that is formed by plasma ashing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a conductivepattern such as electrodes on a compound semiconductor layer, and moreparticularly, to a method of forming a fine electrode pattern using alift-off process.

2. Description of the Related Art

In a technical field of an optical communication and optical recording,the demand of using a surface emitting semiconductor laser (VerticalCavity Surface Emitting Laser Diode) has been increased because offacilitation of the two-dimensional light source array. The surfaceemitting semiconductor laser device is typically made of compoundsemiconductor layers such as GaAs, AlGaAs and so on, and generally thelift-off process is employed to form electrodes and wirings with apredetermined shape and size.

For example, Japanese Laid-Open Patent Application No. 10-154707discloses a lift-off process. As shown in FIGS. 10A to 10E, an upperphotoresist layer 15 is formed on a lower photoresist layer 13 and theupper photoresist layer 15 is insolubilized after patterning of twophotoresist layers. Then the lower photoresist layer 13 is dissolved toform an undercut for the upper photoresist layer 15 n. After that, aconductive film (unshown) is formed by the lift-off process. By theformation of the undercut, the needed time for the lift-off process isshortened and the generation of flashes around the conductive layer isprevented.

However, there are following problems to be resolved in forming thepattern by the lift-off process as shown in the above application. Whenthe electrodes and/or the metal wirings pattern are formed on thecompound semiconductor layer such the surface emitting semiconductorlaser device, the use of an alkali developer for forming the undercut inthe lower resist layer causes the contact with the surface of thecompound semiconductor layers and causes them to be etched. The compoundsemiconductor layer, e.g. GaAs, has a surface that is chemically weak,and therefore the etching progressives clearly memorably and the lifetime of devices may be shortened by the surface damages caused by theetching and the performances may be affected by the damages.Furthermore, when a p-side electrode which defines an emission windowfor the laser light is formed the contact layer such GaAs in the surfaceemitting semiconductor laser, the etching of a contact layer has anadverse effect on the optical characteristics or properties of the laserlight.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand provides a method of forming a conductive pattern of electrodes orwirings in which the surface damages on the compound semiconductor layeris prevented as far as possible.

Furthermore, the present invention provides a method of forming aconductive pattern in which a fine pattern of electrode or wirings canbe formed on the compound semiconductor layer by means of the lift-offprocess.

Furthermore, the present invention provides a method of manufacturingthe surface emitting semiconductor laser using the method of forming theconductive pattern.

Furthermore, the present invention provides a surface emittingsemiconductor laser and its manufacturing method in which the harmfuleffects on the optical characteristics of laser light are inhibited andthe life time of device is improved.

According to an aspect of the present invention, a method of forming aconductive pattern such as an electrode on a compound semiconductorlayer has forming a first organic layer on the compound semiconductorlayer, forming a second layer on the first organic layer, the secondlayer being resistant to a plasma ashing, forming a pattern including afirst aperture in the second layer, forming a second aperture in thefirst organic layer by the plasma ashing of the first organic layerusing a mask of the pattern including the first aperture to expose thecompound semiconductor layer in the second aperture, depositing aconductive layer over a region including the compound semiconductorlayer exposed in the second aperture and the second layer, forming theconductive pattern on the compound semiconductor layer by a lift-offprocess.

According to another aspect of the present invention, a method offorming a conductive pattern over a compound semiconductor layer hasforming a first organic layer on the compound semiconductor layer,forming an inorganic layer on the first organic layer, forming a secondorganic layer on the inorganic layer, forming a first pattern includinga first aperture in the second organic layer, forming a second patternincluding a second aperture in the inorganic layer by etching theinorganic layer using a mask of the first pattern including the firstaperture, forming a third aperture in the first organic layer by aplasma ashing of the first organic layer using a mask of the secondpattern including the second aperture to expose the compoundsemiconductor layer in the third aperture, depositing a conductive filmover a region including the compound semiconductor layer exposed in thethird aperture and the second organic layer, and forming the conductivepattern on the compound semiconductor layer by a lift-off process.

According to another aspect of the present invention, a method ofmanufacturing a surface emitting semiconductor laser, the surfaceemitting semiconductor laser including a first semiconductor mirrorlayer of first conductivity type over a substrate, a current confininglayer over the first semiconductor mirror layer, an active region overthe first semiconductor mirror layer, a second semiconductor mirrorlayer of second conductivity type over the active region and a contactlayer including a compound semiconductor layer over the secondsemiconductor mirror layer, the method has following steps; forming afirst organic layer on the contact layer, forming a second layer on thefirst organic layer, the second layer being resistant to a plasmaashing, forming a pattern including a first aperture in the secondlayer, forming a second aperture in the first organic layer by theplasma ashing of the first organic layer using a mask of the patternincluding the first aperture to expose the contact layer in the secondaperture, depositing a conductive layer over a region including thecontact layer exposed in the second aperture and the second layer, andforming a conductive pattern on the contact layer by a lift-off process.

According to another aspect of the present invention, a surface emittingsemiconductor laser has a substrate, a first mirror formed over thesubstrate, the first mirror including semiconductor layers of firstconductivity type, a second mirror formed over the substrate, the secondmirror including semiconductor layers of a second conductivity type, aactive region disposed between the first and second mirrors, a currentconfining layer disposed between the first and second mirrors, acompound semiconductor layer formed over the second mirror, and anelectrode formed on the compound semiconductor layer. A surfaceroughness of the compound semiconductor layer in at least a peripheralportion of the electrode is not more than 5nm.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail based on the following figures, wherein:

FIGS. 1A to 1E show steps for forming a metal pattern includingelectrode-like according to a first embodiment of the present invention;

FIGS. 2A through 2F show a step for forming a metal pattern includingelectrode-like according to a second embodiment of the presentinvention;

FIG. 3 shows a cross sectional view of the surface emittingsemiconductor laser;

FIGS. 4A to 4C show steps for manufacturing the surface emittingsemiconductor laser according to a third embodiment of the presentinvention;

FIGS. 5D to 5F show steps for manufacturing the surface emittingsemiconductor laser according to the third embodiment of the presentinvention;

FIGS. 6G to 6I show steps for manufacturing the surface emittingsemiconductor laser according to the third embodiment of the presentinvention;

FIGS. 7J to 7L show steps for manufacturing the surface emittingsemiconductor laser according to the third embodiment of the presentinvention;

FIGS. 8M to 8O show steps for manufacturing the surface emittingsemiconductor laser according to the third embodiment of the presentinvention;

FIG. 9P shows steps for manufacturing the surface emitting semiconductorlaser according to the third embodiment of the present invention;

FIGS. 10A to 10E show steps for forming the conductive pattern includingelectrode-like in a prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given of a method of forming a conductivepattern according to an embodiment of the present invention. FIG. 1shows the method of forming an electrode on a compound semiconductorlayer according to the first embodiment of the present invention.

In FIG. 1, a photoresist is provided on a GaAs substrate 100. Thethickness of the photoresist is approximately 1.0˜2.0 μm, which forms alower photoresist layer 101. Subsequently, the lower photoresist layer101 is subjected to a baking process at an appropriate temperature, e.g.130° C. By the baking process, the photoresist is insolubilized and thephotoresist turns to be resistant to a developer.

Next, a photoresist which is resistant to an oxygen plasma is formed onthe lower photoresist layer 101. For example, the photoresist may be aphotoresist containing silicon (FH-SP: production of Fuji Shashin FilmArch Co.). The thickness of the photoresist is approximately 1 μm, whichforms an upper photoresist layer 102.

The upper photoresist layer 102 is subjected to a pre-baking process,for example 90 degree Celsius, and to an exposure process using adesirable mask pattern. Then the photoresist is developed by an alkalideveloper, as shown in FIG. 1B, a pattern including an aperture 103(hereinafter referred to as aperture pattern) is formed in the upperphotoresist layer 102.

Subsequently, as shown in FIG. 1C, the exposed lower photoresist layer101 is ashed by the oxygen plasma by the oxygen plasma etcher using amask of the aperture pattern 103 of the upper photoresist layer 102,thereby the exposed region being removed. By selecting a suitable ashingtime, an aperture pattern 104, which is similar to the aperture pattern103 in the upper photoresist layer 102, is formed in the lowerphotoresist layer 101. Simultaneously, the upper photoresist layer 102is not removed at all since it is resistant to the oxygen plasma. Also,the aperture pattern 104 formed in the lower photoresist 101 is removedlaterally by the oxygen plasma ashing, which forms an undercut structureto the upper photoresist layer 102. The distance S1 of the undercut isapproximately 1 μm.

As shown in FIG. 1D, a metal film is deposited or vaporized over aregion, which includes the upper photoresist layer 102 and the substrateexposed by the aperture pattern 104. The metal film 106 is preferablyplaced within the aperture pattern 104 because the aperture pattern 104is made to the undercut structure to the upper photoresist layer 102.

As shown in FIG. 1E, the lower photoresist layer 101 and the upperphotoresist layer 102 are dissolved for removing, and a conductivepattern 106 such as an electrode and wiring pattern is formed on thesurface of the substrate 100 by the lift-off process.

Now the second embodiment of the present invention will be explained.FIG. 2 shows steps of forming an electrode of the second embodiment ofthe present invention. As shown in FIG. 2A, a photoresist is provided ona GaAs substrate 200 for forming a lower photoresist layer 201 with athickness of approximately 1.0˜2.0 μm. Then the lower photoresist 201 isbaked at a desirable temperature. After that, an inorganic film 202, forexample ITO, which is resistant to the oxygen plasma, is deposited onthe entire surface with a thickness of approximately 1 μm. Additionally,a photoresist is coated on the inorganic film 202 to form an upperphotoresist layer 203 with a thickness of approximately 1 μm, and thenthe upper photoresist layer 203 is subjected to the pre-baking process.

As shown in FIG. 2B, an aperture pattern 204 is formed after an exposureprocess made to the upper photoresist layer 203 using a desirable maskpattern. After that, a post baking process is done.

Next, as shown in FIG. 2C, the inorganic film 202 is etched by anetchant such as hydrochloric acid using a mask of the aperture pattern204 of the upper photoresist layer 203. The region exposed by the upperphotoresist layer 203 is removed off and an aperture pattern 205 isformed in the inorganic layer 202.

Subsequently, the lower photoresist layer 201 is subject to the ashingprocess of oxygen plasma by the oxygen plasma etcher using a mask of theaperture pattern 205 of the inorganic film 202. The inorganic film 202is not etched off substantially because of being resistant to the oxygenplasma or oxygen radical. By performing the oxygen plasma ashing for asuitable time, an aperture pattern 206, which is similar to the aperturepattern 205 of the inorganic film 202, is formed in the lowerphotoresist layer 201, this resulting in a preferable undercutstructure. In this embodiment, the lateral undercut distance S2 of thelower photoresist layer 201 is approximately 1 μm.

As shown in FIG. 2E, a metal film 207 is deposited on the entire surfaceof the substrate. The metal film 207, is placed on the upper photoresistlayer 203 and on the substrate 200 exposed by the aperture 206 of thelower photoresist layer 201.

Finally, the photoresist layers and inorganic film are removed by thelift-off process for forming the metal pattern 208

As described above, by forming the metal pattern using the lift offprocess as shown in FIGS. 1 and 2, the surface of the substrate is notexposed and not etched by the developer and is kept flat so that thedesirable metal wiring and/or electrode pattern can be formed. It isappreciated that a device having good characteristics can be fabricated.

Now a method of manufacturing a surface emitting semiconductor laserwhich is applied by the above process in FIG. 1 will be explained. FIG.3 shows a cross sectional view of the surface emitting semiconductorlaser and FIGS. 4 to 9 show the steps for manufacturing it.

The surface emitting laser 20 of the embodiment is a type of selectiveoxidation surface emitting laser, which includes a laser element 20 of acylindrical mesa structure (post structure or pillar structure). In thedrawing, a protective film on the laser element 20 or mesa structure 21and a bonding pad extended from a metal contact layer are omitted.

Referring to FIG. 4, 1 is a GaAs substrate of n type, 2 is a GaAs bufferlayer of n type formed on the substrate 1. 3 is a lower DBR(DistributedBragg Reflector) layer of n type. 7 is an active region formed on thelower DBR layer 3. The active region 7 is composed of laminated layers,which include an undoped lower spacer layer 4, undoped quantum welllayer 5, and undoped upper spacer layer 6. 8 is a current confininglayer formed on the active region 7. The current confining layer 8includes an AlAs portion 8 a which defines a circular aperture at thecenter and includes an oxidized region 8 b around the AlAs portion 8 a.The oxidized region 8 b confines a current and light which pass throughthere. 9 is an upper DBR layer of p type formed on the current confininglayer 8. 10 is a GaAs contact layer of p type formed on the upper DBRlayer 10. 11 is a p-side contact electrode of annular-shape orring-shape on the contact layer 10, which defines an emission window 11a for the laser light. 12 is a protective film for protecting theemission window 11 a, formed on the contact electrode 11. 13 is aninterlayer insulating film 13 which covers a top end, side and bottom ofthe mesa 21. 14 is a p-side wiring electrode, which is formed on theinterlayer insulating film 13 and is connected to the contact electrode11 though a contact hole 13 a.

The emission window 11 a is a circular shape and its center coincideswith an optical axis which extends through a center of the mesa 21 andextends in a vertical direction with respect to the substrate 1. Thecenter of AlAs portion 8 a of the current confining layer 8 alsocoincides with the optical axis substantially. That is, the AlAs portion8 a and the emission window 11 a are aligned mutually.

Now the manufacturing method of the surface emitting semiconductor laser20 will be explained. In FIG. 4A, semiconductor layers are formed on thesubstrate 1 by MOCVD (Metalorganic Chemical Vapor Deposition) or MBE(Molecular Beam Epitaxy). The GaAs buffer layer 2 of n type, the lowerDBR layer 3, the active region 7 which includes the lower spacer layer 4of undoped Al_(0.6)Ga_(0.4)As, a quantum well layer 5 having undopedGaAs well layer and having undoped Al_(0.3)Ga_(0.7)As barrier layer andthe upper spacer layer 6 of undoped Al_(0.6)Ga_(0.4)As, the upper DBRlayer 9 and GaAa contact layer 10 of p type are sequentially laminatedover the substrate 1.

The lower DBR layer 3 is laminated by 35.5 pairs or cycles of n-typeAl_(0.9)Ga_(0.1)As layer and n-type Al_(0.3)Ga_(0.7)As layeralternately. Each layer is λ/4 n_(r) thick where λ is oscillatingwavelength and n_(r) is refractive index of medium. The carrierconcentration of the lower DBR layer 3 is 2×10¹⁸ cm⁻³ after silicondopant is doped. While the upper DBR layer 9 is laminated by 23 cyclesor pairs of p-type Al_(0.9)Ga_(0.1)As and p-type Al_(0.15)Ga_(0.85)Asalternately. Each layer is λ/4 n_(r) thick where λ is oscillatingwavelength and n_(r) is refractive index of medium. The carrierconcentration of the upper DBR layer 9 is 2×10¹⁸ cm⁻³ after carbondopant is doped.

A p-type AlAs layer 8, which is in place of P type Al_(0.9)Ga_(0.1)Asand is served as control layer, is interposed at the lowest layer withinthe upper DBR layer 9. The AlAs layer 8 is λ/4 n_(r) thick and itscarrier concentration is 2×10²⁰ cm⁻³ after carbon dopant is doped. Atransition region which has a medium aluminum composition ratio may beinterposed between Al_(0.9)Ga_(0.1)As layer and Al_(0.15)Ga_(0.85)As ofthe lower DBR layer 3 and upper DBR layer 9. The P type GaAs contactlayer 10 is 20 nm thick and its carrier concentration is 1×10¹⁸ cm⁻³.

As shown in FIG. 4B, the lower photoresist layer 101 with thickness of1.0 to 2.0 μm is formed on the contact layer 10 and it is baked at adesirable temperature, e.g. 130° C., which makes the lower photoresistlayer 101 insolubilized.

The upper photoresist 102 including silicon with an approximately 1 μmthickness is formed on the lower photoresist layer 101 and is pre-baked.After that, the upper photoresist layer 102 is subjected to the exposureprocess using a desired mask pattern and is patterned to form theaperture pattern 103 as shown in FIG. 4C using Alkali developer.Simultaneously, the lower photoresist 101 is not dissolved because it isinsolubilized to the alkali developer. And then the post-bake isperformed.

As shown in FIG. 5D, the exposed lower photoresist layer 101 is ashed bythe oxygen plasma by the oxygen plasma etcher using the mask of thepattern 103 of the upper photoresist layer 102 and the aperture pattern104 which is in shape of an undercut is formed in the lower photoresistlayer 101. The upper photoresist layer 102 is not removed at all sincethe resist including the silicon gives the property of resistant to theoxygen plasma.

As shown in FIG. 5E, a metal film 105 is deposited over a regionincluding the upper photoresist layer 102 and the contact layer 10exposed by the aperture pattern 104. Since the second aperture pattern104 is made to the undercut structure, the metal pattern 105 can beplaced within the second aperture pattern 104.

Subsequently, as shown in FIG. 5F, the lift-off process is done and thecontact electrode 11 is formed on the surface of the contact layer 10.The electrode pattern 11 is inverted from the aperture pattern 104 andturns to p-side contact electrode 11. The p-side contact electrode 11has a ring or annular shape and its inner diameter defines the emissionwindow 11 a for the laser light. For example, the material of thecontact electrode 11 may be selected from at least one of Au, Pt, Ti,Ge, Zn, Ni, In, W and ITO.

Next, the protective film 12 is formed on the contact layer 10 includingthe p-side contact electrode 11 by PCVD (plasma-assisted chemical vapordeposition) as shown in FIG. 6G. For example, SiON, silicon oxidenitride, is deposited to a thickness of 250 nm.

As shown in FIG. 6H, the protective film 12 where a photoresist patternis not covered on is removed. The photoresist pattern is prepared by thephotolithographic process. The patterned protective film 12 is thusformed on the contact electrode 11.

Next, as shown in FIG. 6I, a silicon nitride mask 16 is deposited to athickness of 820 nm on the contact layer 10 including the contactelectrode 11 and the protective film 12, for forming the mesa.

As shown in FIG. 7J, by using a photoresist mask pattern preparedphotolithographically, the mask 16 is patterned at predetermined size.

As shown in FIG. 7K, the semiconductor layers are etched using the mask16 until a part of the lower DBR layer 3 is exposed and the mesa isformed. The etching is a reactive ion etching (RIE) using trichlorideboron and chlorine.

Next, as shown in FIG. 7L, using the wet type of oxidation furnaceintroducing water vapor, the AlAs layer 8 is exposed in water vapor andheated at 360 degree Celsius which causes the selective oxidation of theAlAs layer 8 from the sidewall of the mesa, and the oxidation region 8 bis formed.

Next, as shown in FIG. 8M, the interlayer insulating film 13 is formedon the mesa structure so as to cover the upper surface, side, and bottomthereof. The interlayer insulating film 13 is deposited to a thicknessof 800 nm by PCVD.

As shown in FIG. 8N, by using a photoresist mask pattern prepared by thephoto lithograph, the interlayer insulating film 13 is subjected to thedry etching and the entire surface of the protective film 12 is exposedand a part of the mask 16 is removed. The etching gases are a mixture ofSF6 and O2 which have a selectivity in etching. By this etching, thecontact hole 13 a is formed so as to expose a part of the p-side contactelectrode 11. After that the photoresist mask pattern is removed.

As shown in FIG. 8O, the wiring electrode 14 is patterned into apredetermined size and shape by the lift-off process after wiringmaterials, e.g., a laminate metal of Ti and Au, is deposited thereon.The center portion of the wiring electrode 14 on the upper face of themesa is removed. The removed size may be larger than the emission window11 a defined by the contact electrode 11, and the wiring electrode 14 iscontacted with the contact electrode 11 through the contact hole 13 a.

The n-type GaAs substrate 1 is then polished from the backside thereofuntil it reaches a thickness of 200 nm. As shown in FIG. 9P, materialsfor forming n-side electrode 15 are deposited on the backside of then-type GaAs substrate 1. For example, a laminate of Au/Ge/Ni/Au may beused for the n-side electrode.

As explained above, it is possible to form the desired shape of thep-side contact electrode 11 on the contact layer 10 and to prevent thesurface of the contact layer 10 at the contact electrode and its aroundfrom being etched by the developer. Thus, the damage of the emissionwindow for the laser light can be reduced considerably, thecharacteristics of the laser light are stabilized and highly reliablesurface emitting semiconductor laser can be obtained.

The preferable embodiment of the present invention is described above indetail, however, the present invention is not limited to thespecifically disclosed embodiments, and other embodiments, variationsand modifications may be made without departing from the scope of thepresent invention.

For example, the above embodiment shows the metal pattern that is formedover the GaAs substrate and GaAs semiconductor layers, however, othermaterials of compound semiconductor substrate and layers may be used.Other conductive materials for forming the electrodes may be used.

The above embodiment exemplifies the use of the oxygen plasma (oxygenradical) in the plasma ashing, however, other charged particles and theultraviolet light may be added. Furthermore, although the aboveembodiment exemplifies two layered photoresist structure and two layeredphotoresist structure interposing the inorganic layer, anothercombination of layers which is resistant to and is not resistant to theplasma ashing may be used. Alternatively, another combination of layerswhich has a selectivity and non-selectivity in plasma ashing or etchingmay be used.

As explained the above, according to the present invention, the etchingof the compound semiconductor layer by the developer described in theprior art can be prevented and the desirable conductive pattern can beformed on the compound semiconductor layer in maintaining a plane andclean surface of the compound semiconductor layer because the aperturepattern on the compound semiconductor layer is formed by the plasmaashing when the conductive pattern is formed on the compoundsemiconductor layer. By applying the process of the present inventionwith the manufacturing process of the surface emitting semiconductorlaser using the compound semiconductor layers, it is possible to makethe desirable pattern of the contact electrode on the compoundsemiconductor layers, consequently, the damage on the surface of thecompound semiconductor layers can be reduced considerably and theoperation of laser light is stabilized and the reliability can beimproved.

Finally, the aforementioned description of the present invention issummarized as follows.

According to one aspect of the invention, a method of forming aconductive pattern such as an electrode on a compound semiconductorlayer includes forming a first organic layer on the compoundsemiconductor layer, forming a second layer on the first organic layer,the second layer being resistant to a plasma ashing, forming a patternincluding a first aperture in the second layer, forming a secondaperture in the first organic layer by the plasma ashing of the firstorganic layer using a mask of the pattern including the first apertureto expose the compound semiconductor layer in the second aperture,depositing a conductive layer over a region including the compoundsemiconductor layer exposed in the second aperture and the second layer,and forming the conductive pattern on the compound semiconductor layerby a lift-off process.

According to this, the surface of the compound semiconductor layer isprevented from contacting with a solution such as an alkalideveloper-like, thus the surface etching is preferably inhibited and thesurface damage by the etching can be reduces.

According to another aspect of the invention, a method of forming aconductive pattern over a compound semiconductor layer includes forminga first organic layer on the compound semiconductor layer, forming aninorganic layer on the first organic layer, forming a second organiclayer on the inorganic layer, forming a first pattern including a firstaperture in the second organic layer, forming a second pattern includinga second aperture in the inorganic layer by etching the inorganic layerusing a mask of the first pattern including the first aperture, forminga third aperture in the first organic layer by a plasma ashing of thefirst organic layer using a mask of the second pattern including thesecond aperture to expose the compound semiconductor layer in the thirdaperture, depositing a conductive film over a region including thecompound semiconductor layer exposed in the third aperture and thesecond organic layer, and forming the conductive pattern on the compoundsemiconductor layer by a lift-off process.

According to this, the surface of the compound semiconductor layer issignificantly prevented from the damages by the etching, which providesthe desirable electrodes and/or wirings on the surface.

In a preferable first method, a photoresist is coated on a GaAssubstrate (or GaAs layer) to form an lower resist layer of an organiclayer. Then the lower photoresist is insolubilized to the developerafter the baking process. Next, a photoresist which is resistant to anoxygen plasma is formed as a upper photoresist layer. The upperphotoresist layer is patterned by a desirable mask pattern prepared bythe photo lithographic process. Next, using an oxygen plasma either, thelower photoresist layer exposed by mask pattern of the upper photoresistlayer is ashed by the oxygen plasma for patterning the lower photoresistlayer. By executing the oxygen plasma ashing at desirable time, thepattern which is similar to the pattern of the upper photoresist layeris formed in the lower photoresist layer. After that, a conductive filmsuch as a metal is deposited on an entire surface. Finally, the lowerphotoresist layer is subjected to the lift-off process to formelectrodes and/or metal wirings on the substrate surface.

In a preferable second method, a photoresist is coated on a GaAssubstrate (or GaAs layer) to form an lower resist layer as an organiclayer. Next, the lower photoresist layer is solidified by the bakingprocess-like and an inorganic layer which is resistant to the oxygenplasma is deposited on an entire surface. Additionally, a photoresistlayer is coated to form an upper photoresist layer. After that, theupper photoresist layer is patterned using a desirable mask prepared bya photo lithograph process. Next, the inorganic layer is etched usingthe mask pattern of the upper photoresist layer. Next, the lowerphotoresist layer is ashed by the oxygen plasma using the mask patternof the inorganic layer. By carrying out of the oxygen plasma process ata suitable time, a pattern which is same pattern or an undercut patternof the upper photoresist layer is formed in the lower resist layer.Next, a conductive film such as metal-like is deposited on an entiresurface. Finally, the lower photoresist layer is lifted off to makemetal electrodes or metal wirings on the substrate surface.

Since the conductive pattern such as metal or wiring is formed on thesurface of the compound semiconductor layer (including the substrate)such as GaAs by using the process according to the present invention,the surface of the compound semiconductor layer is inhibited fromcontacting with the alkali developer-like directly during themanufacturing processes. Consequently, the formation of metal wirings orelectrodes can be realized without etching or damaging the substratesurface such as GaAs, whose surface is chemically weak.

The process according to the invention can be applied with amanufacturing method of the surface emitting semiconductor laser. Themethod of manufacturing a surface emitting semiconductor laser whichincluded a first semiconductor mirror layer of first conductivity typeover a substrate, a current confining layer over the first semiconductormirror layer, an active region over the first semiconductor mirrorlayer, a second semiconductor mirror layer of second conductivity typeover the active region and a contact layer including a compoundsemiconductor layer over the second semiconductor mirror layer, and themethod includes forming a first organic layer on the contact layer,forming a second layer on the first organic layer, the second layerbeing resistant to a plasma ashing, forming a pattern including a firstaperture in the second layer, forming a second aperture in the firstorganic layer by the plasma ashing of the first organic layer using amask of the pattern including the first aperture to expose the contactlayer in the second aperture, depositing a conductive layer over aregion including the contact layer exposed in the second aperture andthe second layer, and forming a conductive pattern on the contact layerby a lift-off process.

According to this, damages on the surface of the contact layers issignificantly prevented and an occurrence of harmful effects on theoutput property of the laser light is prevented.

Preferably, the manufacturing method further includes a step of formingmesa structure which extends from at least the contact layer to thecurrent confining layer and a step of selectively oxidizing a part ofthe current confining layer from the sidewall of the mesa structure. Themanufacturing method can be applied with the surface emittingsemiconductor laser having the mesa structure.

According to another aspect of the invention, a surface emittingsemiconductor laser includes a substrate, a first mirror formed over thesubstrate, the first mirror including semiconductor layers of firstconductivity type, a second mirror formed over the substrate, the secondmirror including semiconductor layers of a second conductivity type, aactive region disposed between the first and second mirrors, a currentconfining layer disposed between the first and second mirrors, acompound semiconductor layer formed over the second mirror, and anelectrode formed on the compound semiconductor layer, wherein a surfaceroughness of the compound semiconductor layer in at least a peripheralportion of the electrode is not more than 5 nm.

The reason why the roughness is not more than 5 nm, the surface of thecompound semiconductor layers is not etched by the solution such asalkali developer. In case the aperture or opening is formed in thephotoresist layer using the alkali developer described as in the priorart, the surface of the compound semiconductor layer is etched, whichgenerally causes depths over 5 nm. Consequently, the surface emittingsemiconductor laser according to the invention makes the surfaceroughness to be planer than that of the prior art, which prevents thedegradation or aggravation of the performances and optical properties ofthe surface emitting semiconductor laser.

Preferably, the compound semiconductor layer includes GaAs layer of thesecond conductivity type. Preferably, the metal electrode is formed bythe lift-off process of the photoresist layers, which are patterned onthe compound semiconductor layer by the plasma ashing.

1. A surface emitting semiconductor laser comprising: a substrate; afirst mirror formed over the substrate, the first mirror includingsemiconductor layers of first conductivity type; a second mirror formedover the substrate, the second mirror including semiconductor layers ofa second conductivity type; an active region disposed between the firstand second mirrors; a current confining layer disposed between the firstand second mirrors; a compound semiconductor layer formed over thesecond mirror; an electrode formed on the compound semiconductor layer;and a protective film that covers the compound semiconductor layer andpartially covers the electrode, wherein the electrode formed by alift-off process utilizes an opening-pattern which is formed by plasmaashing, and surface roughness of the compound semiconductor layer afterformation of the electrode is not more than 5 nm.
 2. The surfaceemitting semiconductor laser as claimed in claim 1, wherein the compoundsemiconductor layer includes a GaAs layer of the second conductivitytype, and the second mirror underlying the GaAs layer includes an AlGaAslayer.
 3. The surface emitting semiconductor laser as claimed in claim1, wherein the electrode is formed in a ring-shaped pattern on thecompound semiconductor layer and wherein an inner diameter of thering-shaped pattern defines an emission window for laser light.
 4. Thesurface emitting semiconductor laser as claimed in claim 3, wherein theprotective film covers the emission window.
 5. The surface emittingsemiconductor laser as claimed in claim 1, wherein the electrode isdefined through the steps of: forming a first organic layer on thecompound semiconductor layer; forming a second layer on the firstorganic layer, the second layer being resistant to plasma ashing;forming a pattern including a first aperture in the second layer;forming a second aperture in the first organic layer by plasma ashing ofthe first organic layer using a mask pattern including the firstaperture to expose the compound semiconductor layer in the secondaperture; and depositing a conductive layer over a region including thecompound semiconductor layer exposed in the second aperture and thesecond layer; wherein forming the pattern in the second layer includes alift-off process, and surface roughness of the compound semiconductorlayer after formation of the electrode is not more than 5 nm.
 6. Thesurface emitting semiconductor laser as claimed in claim 1, furthercomprising an insulation film, wherein the compound semiconductor layeris totally covered with the electrode, the protective film and theinsulation film.